A transformer has become an essential electronic component for various kinds of electric appliances. Referring to FIG. 1, a schematic exploded view of a conventional transformer is illustrated. The transformer 1 principally comprises a magnetic core assembly 11, a bobbin 12, a primary winding coil 13 and a secondary winding coil 14. The primary winding coil 13 and the secondary winding coil 14 are overlapped with each other and wound around a winding section 121 of the bobbin 12. A tape 15 is provided for isolation and insulation. The magnetic core assembly 11 includes a first magnetic part 111 and a second magnetic part 112. The middle portion 111a of the first magnetic part 111 and the middle portion 112a of the second magnetic part 112 are embedded into the channel 122 of the bobbin 12. The primary winding coil 13 and the secondary winding coil 14 interact with the magnetic core assembly 11 to achieve the purpose of voltage regulation.
Since the leakage inductance of the transformer has an influence on the electric conversion efficiency of a power converter, it is very important to control leakage inductance. Related technologies were developed to increase coupling coefficient and reduce leakage inductance of the transformer so as to reduce power loss upon voltage regulation. In the transformer of FIG. 1, the primary winding coil 13 and the secondary winding coil 14 are overlapped with each other and wound around the bobbin 12. As a consequence, there is less magnetic flux leakage generated from the primary winding coil 13 and the secondary winding coil 14. The primary winding coil 13 and the secondary winding coil 14 are overlapped with each other and wound around a winding section 121 of the bobbin 12. Under this circumstance, sine the coupling coefficient is increased, the leakage inductance of the transformer is reduced (e.g. less than 10 μH) and the power loss upon voltage regulation is reduced, the electric conversion efficiency of a power converter is enhanced.
In the power supply system of the electric products in the new generation, for example LCD televisions, the transformer with leakage inductance prevails. The current generated from the power supply system will pass through a LC resonant circuit composed of an inductor L and a capacitor C. The inductor L is provided from the primary winding coil of the transformer. Meanwhile, the current with a near half-sine waveform will pass through a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) switch. When the current is zero, the power MOSFET switch is conducted. After a half-sine wave is past and the current returns zero, the switch is shut off. As known, this soft switch of the resonant circuit may reduce damage possibility of the switch and minimize the noise.
In order to increase the leakage inductance of the transformer, the primary winding coil should be separated from the secondary winding coil by a certain distance to reduce the coupling coefficient of the transformer. Referring to FIG. 2, a schematic exploded view of a transformer with leakage inductance according to prior art is illustrated. The transformer 2 principally comprises a magnetic core assembly 21, a bobbin 22, a primary winding coil 23, a secondary winding coil 24 and a tape 25. The bobbin 22 comprises a first side plate 221, a second side plate 222 and a winding member 223. The tape 25 is wound around the middle portion of the winding member 223 and has a width d. The winding member 223 is divided into a first winding section 2231 and a second winding section 2232, which are located at bilateral sides of the tape 25. The primary winding coil 23 and the secondary winding coil 24 are wound around the first winding section 2231 and the second winding section 2232, respectively. For safety regulations, the tape 25 is used for isolation between the primary winding coil 23 and the secondary winding coil 24. As the width d of the tape 25 between the primary winding coil 23 and the secondary winding coil 24 is increased, the coupling coefficient is reduced and the leakage inductance of the transformer is increased. Under this circumstance, the resonant circuit of the power supply system will be conveniently controlled.
Although the transformer of FIG. 2 is advantageous for increasing the leakage inductance, some drawbacks still exist. For example, since the primary winding coil 23 and the secondary winding coil 24 are separated by the tape 25, the coupling coefficient is reduced and the leakage inductance is too high (e.g. greater than 50 μH) to be lowered or adjusted.
As previously described, the transformer of FIG. 1 or FIG. 2 fails to adjust the leakage inductance within an acceptable range.
In views of the above-described disadvantages, the applicant keeps on carving unflaggingly to develop a transformer having adjustable leakage inductance according to the present invention through wholehearted experience and research.